PureRef Download Forum Support
Posts: 9
Hi.
I compared PureRef's greyscale mode with GIMP's Colour -> Desaturate operation, and I noticed a difference:
https://s15.postimg.org/sk9e23257/pure_Ref_GIMP_greyscale.png

GIMP's has more contrast it seems? They're using this:
https://github.com/GNOME/gimp/blob/master/libgimpcolor/gimprgb.h#L182-L198

But usually people use this for luminance:
https://github.com/ImageMagick/ImageMagick/blob/master/MagickCore/pixel-accessor.h#L304-L309

...and provided that the R,G,B values used in that conversion are linear, then converted back to sRGB.
Posts: 9
Oh, I just remembered, someone approximated the greyscale conversion that Photoshop produces. See the "like-Photoshop Formula" section:
http://colaargh.blogspot.com.br/2012/02/readable-text-in-colour.html

They call that function with these weights: brightness_value = gamma_grey(R, G, B, 0.2235, 0.7154, 0.0611);

There's also this reference, they have a repository with shader code:
https://github.com/ElliotMebane/GreyShaders/blob/master/Resources/shaders/LuminanceC.fsh
Admin
Posts: 448
Thanks for all the links! We're using the same method as most of those but with slightly different weights: (R * 11 + G * 16 + B * 5)/32

This is our first iteration on the grayscale feature, we'll look into tweaking it for future versions of PureRef.
Posts: 8
I was also looking into this to see if it's correct (that is if it takes in account color inherent value as well) and I'm happy to see that it does. https://imgur.com/a/9V6hq. Images on the left are desaturated by pureref and ones on right are desaturated in Krita and pasted in pureref for conveneince.

I compared it with Krita's Desaturate Luminosity (ITU-R BT.601) algorithm which is the correct way to grayscale. You can achieve the same result with a black layer on top + color blending mode.
Posts: 9
Try to compare extreme colours like pure red (255,0,0) and pure green (0,255,0), they show the difference better (if there is any difference at all, of course).

In the image you posted there seems to be some subtle difference on the background of the anime style portrait at the bottom.
Cutting and pasting a piece on top of the other:
https://s1.postimg.org/7yh32fxqsf/pure_Ref_lightness_difference.png
Posts: 8
I did test with pure reds & blues and other colors, I just didn't include it in the pic here. Now that you mention it I see there's a very slight difference. But really it's so minimal from the practical point of view I'm ok with it. Not to say that I'm against more tests and possibly fixing it if there's a problem.
Sorry, only registered users may post in this forum.

Click here to login